Topic 3.6: Enzymes
Why Should We Study Enzymes?
 All metabolic reactions such as photosynthesis,
digestion and respiration are catalysed by
enzymes, allow them proceed faster at body
temperature.
 Understanding the nature of enzyme functions
will enable us to understand why metabolic
reactions such as photosynthesis and respiration
are affected by factors such as pH and temperature
 Use of enzymes in biotechnology for: food
production e.g. lactose-free milk; manufacture of
antibiotic penicillin and in biological washing
powders.
Lesson’s objectives:
(i) Define enzyme and active site;
(ii) Explain enzyme–substrate
specificity;
(iii) Explain the effects of:
temperature; pH & substrate
concentration on enzyme activity
(iv) Define denaturation.
What is an enzyme?
 globular protein
Active
site
which functions as
a biological
catalyst, speeding
up reaction rate by
lowering activation
energy without
being affected by
the reaction it
catalyse
What is an Active Site?
Active
Site
 a position on the
surface of an
enzyme molecule
to which
substrate(s) bind.
Mechanism of Enzyme Action
e.g. H2O2
e.g. O2 + H2O
Progress of Reaction
 In an enzymatic reaction, the substrate binds to the
active site forming an enzyme-substrate complex
 The active site can lower activation energy (EA) barrier
by:
(i) Orienting substrates correctly
(ii) Straining substrate bonds thus weakening them
(iii) Providing a favorable microenvironment for
reaction to occur
(iv) Allowing change in molecular shape of the substrate
to occur
Using the enzyme structure, explain the
enzyme-substrate specificity.
 each globular enzyme includes an active site with a specific,




three-dimensional shape which is complementary to the
shape of the substrate
the globular enzyme active site also includes a specific set of
charges which are complementary to the charges of the
substrate
through complementarity of shape and charge, the substrate
is attracted to, and fits precisely into, the active site - this is
known as the lock and key model
causes weakening of bonds in substrate to lower activation
energy.
the enzyme’s active site is not rigid and substrate can induce
slight changes in shape
State the factors that affects
enzyme activity.
 Temperature;
 pH;
 Substrate Concentration;
 Inhibitors
Effect of Temperature on Enzyme
Activity.
b
Enzyme
Activity/%
a
c
d
X
Temperature/o C
(a) Enzyme activity increase with increase in
temperature, temperature increases the kinetic energy
of the substrate and enzyme molecules causing more
collisions of the substrate with the active site and the
formation of activated complex's and product(s).
(b) The optimal temperature (X) is the temperature at
which highest enzyme activity, the rate of reaction is at
it best. There is a compromise between decreasing
enzyme stability and kinetic energy of the reactants.
(c) Temperature above optimum temperature increases
the kinetic energy of the enzyme molecules so much
that they break bonds, change shape of the active site,
enzyme become denatured.
(d) All enzymes are denatured and enzyme activity
stops.
What is Denaturation?
 is a structural change in a protein/enzyme that results
in the loss (usually permanent) of its biological
properties. Once denatured, enzymes are no longer
capable of catalyzing biochemical reactions.
 Denaturation is caused by excess heat and change in
pH from optimum pH
Effect of pH on Enzyme Activity.
Trypsin
Pepsin
Enzyme
Activity/%
r
q
pH
 pH also affects the rate of reaction of an enzyme
catalyzed reaction.
 Each enzyme has an optimal pH at which maximum
rate of reaction is achieved.
 At pH above or below the optimal pH the enzymatic
activity decreases because bonds are made and broken
which change the shape of the active site and therefore
decrease the rate of reaction.
 The two enzyme shown in the sketch graph illustrate
the fact that different enzymes can have very different
optimal pH e.g. Pepsin has an optimum pH of 1.8
while trypsin has an optimum pH of 7.8
Effect of Substrate Concentration on
Enzyme Activity.
b
a
Enzyme
activity/%
Substrate Concentration/arbitrary
Units
 as substrate concentration increases enzyme activity
increases because there are more collisions between
the substrate and the enzyme molecules so that more
activated complex's are formed and therefore product
per unit time
 at high substrate concentration, enzyme reaches
maximum activity and enzyme activity remain
constant with increase in substrate concentration
 All active sites are occupied by substrate molecules at
any one time such that adding more substrate
molecules does not increase the rate of reaction.
Review Questions:
Q1 An enzyme __________________
A
is a source of energy for
biochemical reactions
B
is an organic catalyst
C increases the EA of a reaction
D is a inorganic catalyst
Q2 What name is given to the reactants
in an enzymatically catalyzed reaction?
A products
B active sites
C reactors
D substrate
Q3 As a result of its involvement in a
reaction, an enzyme _____.
A
permanently alters its shape
B
loses energy
C
is used up
D is unchanged
Q4 The active site of an enzyme is the
region that
A
binds allosteric regulators of the
enzyme.
B
is involved in the catalytic reaction
of the enzyme.
C
binds the products of the catalytic
reaction.
D is inhibited by the presence of a
coenzyme or a cofactor.
Q5
Which curve represents the behavior of an
enzyme taken from a bacterium that lives in hot
springs at temperatures of 70°C or higher?
A
B
C
D
E
curve 1
curve 2
curve 3
curve 4
curve 5
Q6 Which curve was most likely generated from
analysis of an enzyme from a human
stomach where conditions are strongly acid?
A
B
C
D
E
curve 1
curve 2
curve 3
curve 4
curve 5
Home Work:
Read your textbook pp 66-69 plus the lesson
notes and answer the following questions:
(a) Outline how enzymes catalyze biochemical
reactions.
[3]
(b)Outline enzyme-substrate specificity.
[5]
(c) Explain the effects of temperature, pH and
substrate concentration on enzyme activity.
[8]
Any Questions?
Will this be in
exam?!
Go and put
digestive
enzymes to
work with
some . . .